The service, in one paragraph
VIES - VAT Information Exchange System - is not a database of its own. It is a front door that passes your query to the national VAT register of the country in question and returns the answer: valid or invalid, and, for most countries, the registered name and address. It is run by the European Commission and free to use.
When you must check
When you sell B2B to another EU country at the zero rate (intra-Community supply), the zero rate is only defensible if your customer’s VAT number was valid at the time of supply. Tax authorities can and do reclaim the VAT from the seller when the number turns out invalid. One check per new customer, repeated periodically for regulars, is the working standard.
What a valid result proves - and what it does not
A valid result proves the number exists and is active for cross-border trade. It does not prove the person ordering actually represents that company. For meaningful amounts, record the consultation number VIES returns as evidence you checked, and match the registered name against your customer record.
Checking one by one, or automatically
The manual route is the Commission’s own site. The scalable route is software: VIES offers a machine interface, so a system can validate a VAT number the moment a customer record is created, store the evidence, and re-check periodically. In VFusion Retail this runs inside customer management for exactly that reason.
Service authority: the European Commission’s VIES portal. ec.europa.eu - VIES